A giant rock-carving machine will soon be rumbling beneath city streets as a massive sewage storage tunnel takes shape.

The city said this week that the tunnel boring machine was being lowered into an access shaft at Chamberlain Avenue and Kent Street and will soon start grinding out the tunnel.

The machine will bore a two-kilometre tunnel underneath downtown Ottawa before emerging near the Supreme Court. Last year, the city awarded a construction contract to a joint venture of infrastructure firm Dragados and Ottawa heavy construction firm Tomlinson for the Combined Sewage Storage Tunnel project, which has a total price tag of $232.3 million.

Late in 2016, shovels first entered the ground to create an access shaft to make way for the tunnel boring machine, which will be 250 metres long once fully assembled and can chew through 20 to 25 metres of rock a day.

City of Ottawa photo.

“The average citizen won’t know that the TBM is working underground,” said Paul McCarney, Tomlinson’s vice-president of business development, in a statement last year. “It will be literally like a mechanical mole working deep in the ground.”

He added that the machine cost in the neighbourhood of $7 million.

The overall project consists of two interconnected tunnels spanning a total of more than six kilometres, aimed at reducing the amount of raw sewage that’s dumped directly into the Ottawa River.

During heavy rainstorms, the tunnels will hold up to 43,000 cubic metres – the equivalent of 18 Olympic-sized swimming pools – of runoff and wastewater until it can be treated.